Ch. 5] ATTERBERG LIMITS 101 



these tests lies in the empirical correlation between field performance 

 and test data. Inquiry as to "why" does not form a necessary part of 

 its application. As an example, the standard "Proctor" compaction 

 test may be cited. This test is used to determine the compaction 

 properties of soil when placed as fill. The basis for this test lies in 

 the well-known fact that a given soil, depending on its moisture con- 

 tent, will compact to different densities with the same amount of roll- 

 ing. The Proctor test is, therefore, a standardized test which deter- 

 mines the optimum moisture content for maximum density by pound- 

 ing into a cylindrical mold with a standard weight, dropped from a 

 standard height, representative soil samples with different moisture 

 contents. This operation simulates the energy transmitted by the 

 tamping of a sheep's-foot roller on the soil in the field. The data from 

 the Proctor compaction test are applied directly in construction, with- 

 out further analysis, merely by specifying that all fill be compacted 

 with the determined optimum moisture content. 



Atteebekg Limits 



With increasing water content a clay changes consistency and passes 

 from a solid state through a plastic state to, finally, a liquid state. 

 Each soil possesses a rather characteristic set of limits to these three 

 states. These limits of consistency are arbitrarily fixed by a standard- 

 ized testing procedure which was first proposed by Atterberg and which 

 has come to be called the Atterberg limits (Terzaghi and Peck, 1948, 

 pp. 32-36) . The water content defining the upper limit of the plastic 

 range is called the liquid limit, and that defining the lower limit is the 

 plastic limit. The numerical difference between these two limits for 

 any soil is the plasticity index of that soil. 



Statistical studies of the Atterberg limits of many clays (Casa- 

 grande, 1947) have shown some interesting relationships among a num- 

 ber of soil properties. It has been noted, for example, that the larger 

 the plasticity index of a soil, the greater is its plasticity, its compressi- 

 bility, and its dry strength. In addition, it has been found that, when 

 the plasticity indices and liquid limits for a large number of clay sam- 

 ples coming from the same bed or from geologically related deposits 

 are plotted on a graph, the data define a straight line (Fig. 1). Fur- 

 thermore, the linear plots of clays of different geologic origin occupy 

 different areas on the graph. It is also noteworthy that all the lines 

 in Fig. 1 are roughly parallel. 



